In the processing of semiconductors, such as GaAs in this example, there are difficulties in measuring the temperature of wafers or of thin layers, especially in applications where the temperature has to be known accurately, and no physical contacts to the wafer (or thin layer) are permitted. Two examples of processes where these problems arise are "heat-cleaning" of wafers prior to growing subsequent layers on them by Molecular Beam Epitaxy, and pre-activation "heat cleaning" of photocathodes. The latter represents the present application of the invention.
A device which has been used in an attempt to overcome these difficulties is the pyrometer, which utilizes the black-body (or "gray body") radiation of the sample in order to measure its temperature. This method, however, is valid only when the wavelength of the radiation used is such that its characteristic coefficient of absorption is very large in comparison with the reciprocal of the thickness of the wafer or the thin layer. Such is rarely the case with wafers or thin layers of semiconductors such as GaAs, since the long wavelength light (.lambda.&gt;1000 nanometers) used in pyrometry is hardly absorbed (if at all) by the semiconductor whose bandgap energy exceeds that of the light. Only for thick wafers, having temperatures well above room temperature, can the pyrometric method be applied: in these situations, wavelengths of about 900 nanometers are employed.
Pyrometers, therefore, when used in applications to GaAs or to semiconductors of comparable bandgaps, almost always monitor the temperature of the body on which the semiconductor wafer rests rather than the actual temperature of the semiconductor material. In the case of the photocathode bonded to a glass faceplate, the pyrometer (utilizing radiation above 900 nanometers) absorbs radiation emitted by the glass faceplate. The cathode itself, which is totally transparent to such radiation, is not "seen" at all by the pyrometer; and, furthermore, the cathode layer introduces an additional complication by acting as a thin film interference filter. This latter effect causes the pyrometric temperature readings of the glass faceplate itself to be in error -- depending on the thickness of the cathode layers. The thinner the cathode layer, the more sensitive the pyrometer reading to small variations in the layers' thickness.
The present invention is based on the monotonic change in the optical absorption coefficient as a function of temperature. In the specific example to which the invention is applied herein, the controlling phenomenon is the narrowing of the bandgap of the semiconductor [it is the direct optical bandgap in the case of GaAs] with increasing temperature. Since the absorption coefficient for light of a narrow spectral range, whose photon energy is slightly higher than the bandgap energy, depends on the separation between these two energies (i.e. the photon energy and the bandgap energy: see Eq. 2), it follows that the absorption coefficient will depend on the temperature of the GaAs wafer or thin layer. The energy of the narrow spectral range employed in this mode must be such that it stays above the band edge at all temperatures of interest [if, at any temperature, the bandgap exceeds the spectral energy, then the light will be transmitted unabsorbed and thus will cease to be a measure of the temperature].
Indeed, this invention is applicable to all materials whose optical absorption coefficient is a monotonic function of temperature. It is applicable, in particular, to all semiconductors and is enhanced by selecting narrow optical spectral ranges very close to the respective bandgaps. The underlying mechanism is the same as detailed in this description of the invention as applied to GaAs: the absorption of optical radiation close to the bandgap [and exceeding the latter's energy by a small amount] is a function of the bandgap. Since in all semiconductors the bandgap is a function of temperature, the invention applies to all semiconductors. It further applies to semiconductors wherein the bandgap is either direct or indirect.
It is the object of the present invention to determine the exact temperature of the semiconductor thin layer or wafer, without contacting it physically. The invention is based on measurement of optical transmission, utilizing a properly selected narrow band spectral range, which undergoes moderately weak absorption as it transits through the semiconductor. That optical transmission depends on the bandgap of the semiconductor medium. The bandgap, in turn, is a function of the temperature of that semiconductor layer within which the absorption takes place. Consequently, the optical transmission depends on the temperature of the layer or the wafer.
The present invention does not only provide a method of accurate determination of the temperature, but it furthermore is employed -- through an electrical feedback loop -- to control said temperature by adjusting the power to the heating agent. In general, an independent and constant light source can provide the light whose absorption in the wafer or layer is used to monitor the latter's temperature and, utilizing a loop to control the separate heating agent, maintain the above temperature at any desired value or values.
In the specific application of the invention to "heat cleaning" the wafer as described here, the heating agent is an incandescent projection lamp emitting white light which heats the wafer by being partly absorbed in it. The method of this invention is applied by selecting a very narrow spectral range of the white light from the projection lamp, and measuring its absorption by the wafer. In other words, the heating of the wafer and the light whose absorption is measured to monitor the temperature are both provided by the same source (the lamp).
Since the intensity of the lamp varies during the "heat-cleaning" process, the application of the invention contains a continuous comparison of the intensities of the light component transmitted through the wafer, with that emitted by the lamp. This "normalization" procedure enables us to separate those changes in transmission through the wafer which are due to the latter's varying temperature, from changes which are due to variations in the light intensity emitted by the lamp.
This invention overcomes all the aforementioned shortcomings of existing pyrometric methods.
These and other objects of the invention are achieved according to the invention by providing a source of optical radiation having a desired spectral component and directing that optical radiation to a layer of material having a bandgap which varies as a function of temperature. The optical radiation transmitted through the layer of semiconductor material is detected and analyzed to determine the optical absorption which has occurred. Due to the relationship between direct bandgap and optical absorption, analysis of the transmitted optical radiation will provide an indication of the direct bandgap of the material which, in turn, is indicative of the material's temperature.
For a semiconductor wafer or layer, an in situ temperature determination may be accomplished while the wafer is in a heating chamber even though the temperature detection apparatus is maintained outside the heating chamber. Of course, the temperature detection apparatus could just as well be wholly or partially within the chamber if it is tolerant of the processing temperatures. A light source, which may double as a heat source, is provided which emits light including light within a spectral range having a photon energy slightly higher than the bandgap energy of the semiconductor. Since the absorption coefficient for this spectral range depends on the separation between the photon energy and the bandgap energy, it is possible to derive information relating to the bandgap by examining the absorption by the GaAs wafer in the spectral range of interest. Additionally, the direct bandgap of GaAs narrows as temperature increases. Thus, information regarding the temperature of the GaAs wafer may be derived from the absorption of the identified spectral range.